Quantum physics describes a universe that is profoundly mysterious. Einstein, arguably the most revolutionary thinker of modern times, struggled greatly with quantum theory. This groundbreaking new perspective, ironically triggered by his own early work, simply didn't fit his views on physical reality. Would quantum theory not have been as successful as it was, Einstein could have brushed it aside. But from the early days, the theory was immensely successful. And no one around him seemed to have any problems with it. Einstein must have felt lonely at times, but he was convinced enough in the power of his own reasoning to persist in his skepticism towards quantum physics.

Einstein was wrong. Still, understanding Einstein's struggles is a prerequisite for grasping the profound weirdness of quantum physics. I would argue that to successfully embrace quantum theory, you have to go yourself through Einstein's struggles, and... leave them behind. Einstein never succeeded at that, but you can!

You can because you have the advantage of living in later years. Would Einstein have lived just ten more years, he would have liberated himself from the shackles that bounded him to classical physics. A stupefying new insight derived by the UK physicist John Bell would probably have given him the shock of his life, but it would have helped him taking the mental hurdle to accept quantum physics as our deepest view on reality. Einstein would probably have felt his famous physics intuition had lost contact with reality, and he would certainly happily have admitted that Feynman's claim "nobody understands quantum physics" makes no exception for him. I would love to hear the words that the most quotable physicist would have uttered at the occasion. Probably something along the lines "Magical is the Lord, magical in subtle and deceitful ways bordering on maliciousness".

Alas, all of this was not meant to be. Einstein lived till 1955. John Bell was working on his PhD in nuclear physics at that time, and not yet involved in what is now referred to as Bell's theorem. As a result, Einstein died as the last of the great classical physicists, unaware of the insights into quantum magic that would soon unfold.

So what was it that this great mind struggled with, and what insights were obtained after his dead?

Lay persons interested in quantum mysteries are invariably presented a dumbed-down version of the arguments against quantum theory put forward by Einstein. They learn about Heisenberg's uncertainty principle and the probabilistic nature of quantum measurements, and are subsequently presented with Einstein's quote "God doesn't play dice". This presents you with no more than a caricature of Einstein's position on quantum mechanics. Einstein's concerns ran much deeper and addressed the question what constitutes reality in an entangled quantum world. Einstein talked about God and dice, but is also quoted to have asked the question "Do you really believe the moon is there only when you look at it?". And that quote brings us much closer to Einstein's skepticism towards quantum theory.

Spooky Socks

Once upon a time in a small university town, a natural philosopher called Albert filled his days contemplating life, the universe and everything. Like many of his colleagues, Albert struggled each morning to equip his feet with a matching pair of socks. Would you spot Albert on a number of days, chances are you would occasionally observe his shoes filled with a red left foot and a green right foot or any other combination of colors.

Although his absent-mindedness was strong enough to serve as explanation for any ill-fitting garments, Albert did have a valid excuse for his poor choice of outfit. His Danish housekeeper, Niela Bohr, kept his socks in a chest of drawers. Three rows, each consisting of three drawers, made up this piece of furniture. Whenever Albert pulled open a drawer in search for socks to wear, he would be presented either a pair of matching socks or a single sock. Every subsequent drawer he opened, would reveal socks of a color different from those in the drawers already opened. To make things worse, each drawer opened would block from opening all drawers not in the same row and all drawers not in the same column. This effectively limited Albert each morning to the opening of three drawers configured in a horizontal row or in a vertical column.

Each night Niela prepared the chest of drawers for the next morning. To Albert's frustration, he couldn't figure out what procedure Niela followed. Each morning when opening a line of three drawers, the outcome came to him as a compete surprise. Albert labeled the drawer rows A, B and C, and the columns X, Y and Z, and started recording his observations. Each morning he wrote down a line like B121, indicating the opening of the drawers in row B containing 1, 2, and 1 socks respectively.

Following a few weeks of observations, Albert has recorded the following set of data:

When questioning Niela about the way she filled the chest of drawers each day, she responded that she didn't fill the drawers, rather she prepared them according to the laws of quantum physics. "What do you mean you don't fill the chest of drawers?" Albert asked, "surely you fill it as I have never encountered an empty drawer." Niela hesitated. "Sir, this is a quantum chest. There is no reality associated with the contents for each drawer." Albert looked puzzled. "You mean the unopened drawers don't contain any socks?" Albert focused at her face. Was she making a joke? She seemed perfectly serious. "Sir, an observation not made is a non-existent observation. Now if sir would please excuse me, I need to wash sir's socks for tomorrow and prepare sir's chest of drawers." And off she went.

Albert thought about Niela's puzzling remarks. It all didn't make sense. He knew about this weird quantum theory. A statistical theory that he was sure, could not represent the deepest truth of nature. He knew for a fact that each time all drawers are filled. If that was not the case, surely he would on occasions have hit an empty triplet of drawers. There must be some explanation. Probably she was playing a game with him, and filling the drawers according to some secret allocation algorithm.

Months go by, the list of drawer observations kept growing, but Albert didn't manage to work out the algorithm. One day, he explains the issue to his colleague, Jim Bell. Jim was a practical guy and an expert on quantum theory. "Can I have a look at the data?", he asked. Albert handed over a sheet of paper. It took Jim only a few seconds to remark "This is interesting, a horizontal line of drawers always contains an even number of socks, while a vertical line always contains an odd number of socks". He handed back the paper to Albert, who once more inspected the data. His mouth opened. With his eyes wide open and still fixed on the paper, he uttered "But this is impossible". Jim smiled, "Well, the results are puzzling indeed. But those are your own observations. If you doubt them, you have to redo them."

Albert was still staring at the paper, and didn't look up. "This really is impossible. If at any given morning I would open three rows, I would end up with an even number of socks. But would I open three columns I would end up with an odd number of socks. Yet in both cases I would have opened the same nine drawers. This is absolutely impossible."

"Right. Albert, can I remind you that you started by telling me that chest of yours contains quantum drawers and that on each given day you can open only one row or one column of drawers at a time?" "Yes, but let's assume, just for sake of argument, that we can open all drawers." "Albert, you have to make up your mind. Can you, or can you not open all drawers? If not, then you should realize it is not that you don't know the facts about the contents of the drawers that can not be opened, there simply aren't any such facts."

Hours later, back at home Albert was staring at his spooky drawers. He had checked the data many times. There was no doubt, Jim's observation on even and odd sock counts was correct. Jim had tried to convince him it is meaningless to discuss the contents of drawers that can not be opened. But still, a-priori there is no drawer that can not be opened. Each morning he can decide to open any of the nine drawers, it is just that already opened drawers limit the opening of subsequent drawers. So each drawer must contain either one or two socks. Or not? This quantum stuff was really driving him crazy.

Could it be that the chest contained a hidden mechanism that played tricks on him? Maybe the socks could move from one drawer into the other based on the drawers that he opened. The next few mornings Albert checked the drawers that he pulled open and inspected them for any hidden mechanics or other tricks. Nothing of that. There was no way for the socks to move from one drawer to the other.

Could it be that Niela knows in advance if he was going to select a row or a column of drawers? No, this is a crazy thought. Precognition is pseudoscientific nonsense. But physical reality not allowing him to talk about the contents of unopened drawers seemed even crazier. So what the heck. Albert took a die and marked it with the symbols A, B, C, X, Y and Z. Henceforth, each morning he threw the die and opened the row or column of the cupboard corresponding to the symbol on the die.

Basing the choice of the drawers to be opened on the throw of a die didn't change anything to the outcomes. Rows continued to come up with even numbers of socks, and columns with odd numbers of socks. Albert looked again at the chest of drawers. What a spooky device! A spooky and revealing cupboard that was telling him something deep about the nature of physical reality. His observations on drawer contents did not leave room for any other explanation than what Jim was telling him all along: we are living in an utterly strange quantum universe. A universe in which what could have happened but didn't has no meaning.

Emergent Quantum Mechanics

What is one to make of all this? Is it possible to prepare the chest of drawers in such a way that even rows and odd columns result? Like it or not, according to quantum theory, the answer is yes. Albert's chest of drawers can in principle be constructed. And what's more: similar such devices have recently been created and operated. This is done not with drawers filled with socks, but rather with entangled microscopic systems such as photons in quantum optical experiments. The principle is all the same.

Einstein considered quantum theory as nothing more than what in our current terminology would be called an emergent theory. An approximate theory that resulted from an underlying more fundamental truth. That fundamental truth, Einstein felt, must honor the existence of physical reality independent of the measurements one executes. Einstein was convinced that things one cannot know anything about (such as the number of socks in a drawer that can not be opened) do exist all the same.

Most of Einstein's contemporaries considered Einstein's thoughts about the existence of an objective reality as philosophical musings without any practical consequences. Wolfgang Pauli was very clear on this when he wrote: “One should no more rack one's brain about the problem of whether something one cannot know anything about exists all the same, than about the ancient question of how many angels are able to sit on the point of a needle. But it seems to me that Einstein's questions are ultimately always of this kind.”

Bell's theorem published in 1964 by John Bell, and particularly its extension published a few years later by Simon Kochen and Ernst Specker, establish that Einstein and Pauli were both wrong. The question if something one can not observe does exist, is not a meaningless philosophical musing, but a question that can be answered experimentally. And when these experiments are performed, that is: when devices like Albert's chest of drawers are build and operated, quantum entanglement effects as displayed by Albert's socks come to the fore. These spooky effects force us to answer the question 'does something exist if we can not know anything about it?' with a resounding 'no'. What can not be observed does not exist. This is not a crazy philosophical thought, but a hard experimental fact.

The inevitable conclusion is that if there is a more fundamental truth from which the known laws of quantum physics are emergent, this more fundamental truth must be at least as weird as quantum theory. More in particular, a classical physics theory capable of explaining all of quantum physics - Einstein's hope - can not exist. Any wish for a classical foundation of physics is death, and so is the last great physicist who believed in it. Each scientist is the product of a generation. Einstein's thoughts ran deep, but even he could not stay ahead of developments when a next generation of physicists started to unravel the spookiness of our quantum universe.

A lot more can be said about this, especially if we allow 'free will' to enter the discussion. (Would Albert's drawers be spooky if 'free will' didn't exist?) Yet another potential future blog entry...

please do. I love this topic, maybe because it's so hard to really grasp. This is one of the best explanations of what is happening I've read (together with the one from Brian Greene's *i think it was* Fabric of the cosmos)

Your quasi-quantum system is specifically defined to have outcomes that depend on the orientation of the line of three drawers. One assumes that the rules are the same for everyone.

Choosing the orientation can be made independent of Einstein's decision-making ability if it is random. Any spookiness then depends on the chest managing to move socks around at the last moment, just when the final drawer starts to be pulled open.

All analogies break down at some point, if they did not, they would not be analogies, they would be descriptions.

Would Einstein have lived just ten more years, he would have liberated himself from the shackles that bounded him to classical physics.

I don't think so. Einstein could have known, because already special relativity turns direct realism into a many world setup of consistent past light cones (if you do not want a pseudoscientific Block universe that neglects everything but the gravitational sector). His thinking was direct realistic and would have probably stayed so. There are very clever people also today (e.g. G. 't Hooft), many more years after those "just ten more years", who plainly do not want to let go of direct realism.

Should we not remember Einstien himself is one of the key scientists that brought about the "Quantum revolution"? While he clung to the classical physics probably due in part of his idealistic and social state of mind.

I'm sure Einstien in his time understood more about the true nature of Quantum physics than most of his peers. In the end he did not deny Quantum mechanics had its truths, he simply stuck to his belief it was "flawed".

While ultimately his last years of work to damage te Quantum hypothesis was fruitless, he was still the greatest mind humanity has seen since Newton.

I worked specifically on EPR/Bell type problems as an undergrad, and after a very long time grappling with the issue I've concluded that the only good choice is to take QM full seriously, including its effects on observers, and including decoherence. Once this is done, and its implications are properly worked through, there are no paradoxes left whatsoever, and it allows for a description that is strictly local.

The EPR/Bell experiments are precious hints that force us to acknowledge what otherwise we might find endless excuses to avoid. Some still do prefer to avoid, but I think the arguments are pretty strong that their position is dramatically disfavored and it carries much more baggage....

Instinctively, from the time I first read quantum theory, I've felt that that I was looking at an incomplete painting. Something seems to be missing from the picture, and whatever is missing will probably clear up the confusion about it. IMHO, we probably need a fifth force to plug into our equations, a force yet to be discovered.

Fun article. I only have two kinds of socks, the Gold Toe 513S white, and 513S black. My sock drawer could be adapted to the Stern Gerlach experiment :-)

Einstein certainly understood the math behind quantum field theory, starting with his own contributions to understanding the photo electric effect, and Bohr's quantizing of angular momentum in the hydrogen atom leading to the Balmer formula for spectral lines. I think he was battling with causality, an issue that is central to special relativity.

One could imagine another sock drawer, same number of rows and all. Every time you looked in it, the same socks were there! Like's call this one Newton's chest of drawers. I bet I could build such a piece of furniture myself. The Bohr chest is beyond my own skill set, but I completely accept that it can be built.

At this time, I don't find the emerging work on emergence satisfying. This subject is confusing, emergence is confusing, so it stays confusing, hence little progress for me.

Here is the idea I am playing with these days. Real numbers are a totally ordered set. One can say three things: this comes for that, this comes after that, or this and that are at the same place. I have organized the time in my life this way. I graduated high school before I graduated from MIT. I was in the First Parish Church on the very same day I was married. Time has this quality for any chosen observer: their world line is a totally ordered set in time.

Complex numbers are not a totally ordered set. Picture the complex plane. Should a point in one quadrant be said to be "before" another? One could make up some rules about how to order all things, but then the order depends on those rules. A different set of rules means a different ordering. There is no right ordering.

I have been to many places in my life. These days I mostly go from Acton to Waltham and back, but I go sometimes to Boston and back. How should I organize the places I have been? There is not an obvious one-rule-fits-all.

Systems that are considered classical would be those that use real numbers to describe changes with respect to time. Systems that are considered quantum would need complex numbers with respect to time and space, the space part being the imaginary player. Do calculus with complex numbers to study how things change. Making an observation requires mapping the complex numbers back to the reals so that an observer can place the observation in the middle of their own classical world line. Until the event is mapped to the observer's world line, the event is out there in complex-valued space-time.

Very nice visualization of the Peres-Mermin square. Now try to come up with one for Kochen and Specker's original proof! :P

Of course, if you allowed some embellishment, you could construct a chest showing the behavior you describe; it would only have to have a memory and a means to construct outcomes of sock-drawer openings based on prior sock-drawer openings to do so, essentially equipping it with contextual hidden variables. See, for instance, this paper: http://arxiv.org/abs/1111.3561, which by some strange coincidence I'm going to talk about to our group next week.

Also, as a historical point, Mermin in his paper 'Hidden Variables and the Two Theorems of John Bell' (http://rmp.aps.org/abstract/RMP/v65/i3/p803_1) mentions that Bell actually came up with the Kochen-Specker theorem before he published the one that now bears his name -- which seems somewhat odd, given that Bell's theorem kinda follows from the KS one, by conducting the compatible measurements at a spatially distant point.

With a particle now clocked and confirmed faster than the speed of light, it would seem to contradict this article somewhat in that even if you couldn't observe it, it still exists. If a particle starts at Point A and you are at Point B, you won't be able to see the particle even though it arrives at Point B until it's light (photons) catch up to it. So does the particle exist in that fraction of a second before the light gets there?

I'm getting disturbed reading this. I don't have anything against QM, but many aspects of it as entanglements are a direct result of Einsteins challenges. To me it smacks of Hubris saying that QM is 'right' and Einstein was 'wrong'. The standard theory is wobbling at the moment with no Higgs, or a Higgs in the wrong scale. His later years he was working on five dimensional theory if I remember right, trying to find a 'TOE'. That same foundation he used is nowadays what string theorist use. I'm not impressed by this article.

Well said. Einstein got it wrong but everyone else got it wronger. For 80 years we have had to endure piffling mysticism and nonsensical "interpretations" which are now morphing into an unholy chimera of science and philosophy. Can it get any worse?

Hey, come on, its great fun! I think we are lucky to be experiencing such exciting and baffling times. We are also very lucky to be witnessing some of these exalted scientists' bafflement. Its like watching the Pope saying that maybe Heaven and Hell don't really exist after all.

Thanks all for your thoughtful reactions. One thing that becomes clear from some of the reactions is that I probably should have stressed explicitly in the article that quantum theory has no problems whatsoever in describing what Albert will find in his drawers. From QM perspective Einsteins drawers are rather boring. Non-QM theories on the other hand are logically incapable of describing the contents of the drawers.

@Anonymous : In my eyes Newton and Einstein are the true giants of classical physics. Hey, I am even betting my money on Einstein being correct!

@8-bit : I sympathize with your sentiments, but see Cliff's reaction

@Cliff : I fully agree, and couldn't summarize the state of affairs any better.

@Anonymous : how would an additional force render Einstein's drawers less spooky?

@RRyals : it doesn't matter whether or not QM is complete. Complete or not, QM is the only theory capable of describing Einstein's drawers.

@Doug : I don't think it matters all too much that QM works with complex numbers. What is crucial is that QM utilizes non-commuting objects to describe observables.

@Jochen : thanks for the reference. I was unaware that John Bell had derived a KS theorem. (It does explain why many refer to it as the BKS theorem.) Baffles me why he decided to ignore the (arguably) more general result.

@Lew : Einstein was wrong, but not that wrong... His principle of relativity stands rock solid. There is only one speed, nothing can go faster, nothing can go slower. Once in a while some folks make an error in their velocity measurements. No big deal.

@Yoron : you should be disturbed reading the above (but it is fine not being impressed by the article itself). Whether or not a Higgs turns up at LHC is completely irrelevant. Again: quantum theory has no problems describing Eisntein's drawers. Non-QM theories are logically incapable of doing so. And once more: it is not me telling you Einstein was wrong, it is experiments telling us unanimously.

For you moon-in-the-sky questioners, allow me please to put it this way.The moon is in the sky with or without-you because its CONTEXT (gravitation and history ~ lightcone structure) is going to be there almost no matter what you do. Thanks to the 3+1 structure of spacetime, the moon's orbit is quite stable and will not veer off as a helicopter or the rings of Saturn would disintegrate given more (uncompacted space-like) dimensions. As with the moon, the mass of an electron was established long ago into its context, long before humans came into being.

Same with its spin value: 1/2. No human observer can touch that.

Same with its identity as a fermion.

The direction of its "spin axis" is a different matter altogether. This is one of those "traits" which really depends on your immediate experimental framework ~ context ~ environment ~ yet why confuse it with the moon ... or drawers that are changed every night by a "maid".

Note to blogger: you should have said that at night, the maid visits Bohr's quarters and messes with his drawers. That would have really raised Einstein's fire.

Unsurprizingly, the reactions to articles like the this come from two varieties of readers. Type 1 readers are bothered by the quantum drawers. They seek reasons to deny the validity of quantum theory and even refuse to believe quantum drawers exist. Type 2 (the majority) apear not bothered, but one has to distinguish two subvarieties. Type 2a haven't really read the article, or for other reasons entirely missed the point. Type 2b are not bothered and explain why. They understand Einstein's reservations, but also realize hard facts refute his position. Deep in their heart type 2b persons tend to remain somewhat at unease.

Hey look, we just hooked a Merman. And I was hoping for a Mermaid ... or ... a Merlin. Type 2b ... sounds like an deep astronomic object.

Back to the spirit of the article: let's put it this way >>>

Somethings (like the moon) are determined beforehand.

Somethings (like the direction of a polarization axis) are not.

What were you expecting?

"Quantum's Lesson and Einstein's Dilemma" (copyright) go deeper, and yet, seriously, how much more do we have here than Bohr's circa 1927admonishment that the entire experimental framework is art 'n part of what you get?As an honest capitalist or marketer would say, "if you really want to see something new, you have to MAKE it happen. Don't expect it to be there already, for granted. And once it is there, mind it, feed it."

Details of a show are not set until the very last moments in which they play out.. You do not have to be a producer to know that.And even then (in this virtual age), there is post production editing and spin.

The looseness (free parameters versus those of classical physics) of quantum physics is precisely what you need to step in and make undetermined undefined un-for-granted things happen. We really do live in the best of all possible worlds.

Kind of funny that physicists ~ masters of objective reality~ are reporting this as if it were some new deal. Only a fatalist would be convinced that the last second settings of a stage make no difference.

Nice article. I never really got the details of Bell's equations. Could you explain how the size of the correlations differ between the QM socks chest and a Classical socks chest would differ?

I think Einstein's philosophical problems were also well explained in the EPR paradox.

The crucial point then was the non-locality of QM, which Einstein could not accept. In your socks-drawers example, what is found in one drawer depends what other drawer you looked in before. That I did understand (in a way). The nice thing is that the non-locality in the EPR paradox does not allow you to transmit information faster than light. So it does all fit together with General Relativity.

Sascha Vongehr claimed Gerard 't Hooft somehow was on the side of Einstein. But that is wrong. 't Hooft does really believe in the non-locality of causality (or so he writes), just not in the mysticism of Bohr.

In the end, the non-locality of QM comes back in General Relativity. In the the holography of black bodies. As all of the information in a volume can be described from the surface of the volume, there must be non-local correlations between the states in the volume. There is simply not enough space on the surface to distribute the information otherwise.

Thanks. Not sure what you mean by 'size of the correlation'. The quantum socks drawers result in measurements that always have even rows and odd column, while the best a classical cupboard can do is achieving this on average in 2 out of 3 cases.

This chest of drawers implements the Kochen-Specker theorem, not Bell's theorem. Bell's theorem makes predictions based on the assumpton of a local reality. Kochen-Specker makes predictions based on a non-contextual reality. QM violates both the Bell and the KS predictions. I argue above that the common message of all of this is not so much that QM is non-local, but rather that the assumption of "a reality being out there" can not be maintained.

I agree that Sascha's criticism to 't Hooft's work is misplaced (or to use an expression most prominent in Sascha's vocabulary: a knee-jerk reaction). Gerard is well aware of the Bell and KS resuls, and as far as I know, nowhere did he attempt to 'classicalize' QM (see: http://arxiv.org/abs/0908.3408)

It is the outcomes of observations that could have been made that is 'not out there'. That may sound trivial, but most (if not all) sensible people would agree to the reasoning that if each row in Albert's chest of drawers always results in an even pair of socks, than the full drawer must always contain an even number of socks. That is a demonstrably wrong reasoning given one can open only one row at a time.

Not sure what you mean by "entangled systems not being real". The view that wave functions are real is gaining support recently.

Regarding your link, it says "Whereas many physicists have generally interpreted the wavefunction as a statistical tool that reflects our ignorance of the particles being measured"How would these physicists explain quantum computers? How is a purely statistical wavefunction going to give awesome computational power? The only explanations I have seen for QC's are the wavefunction somehow interfering with itself, or the many worlds interpretation. If you reject the many worlds interpretation, then does that mean the wavefunction's computational power is fundamental to the universe and not explainable in terms of simpler concepts?

They would explain quantum computers just like anyone else. In fact all physicists, regardless their exact interpretation of QM, would agree on the design and results of the QC. A QC surely does not need many worlds to operate properly! In fact, there is no single experimental fact that requires a many world interpretation.

In fact, there is no single experimental fact that requires a many world interpretation.

I understood that decoherence has been effected by a specific microscopic observer/environment systems and the superposition has been put back together, erasing the original observation.

This means that although decoherence took place, a coherent (non-collapsed) superposition still existed after the observation. Since the observer in each such component state must have made the observation, it follows that the superpostion is on multiple observers with different experiences. This is Many Worlds. Of course MW is not observed, it is indeed an interpretation of a superpostion of wolrds each with an amplitude <1 :)

This article perpetuates some recurrent myths about Einstein and quantum mechanics.
In fact, Einstein helped invent quantum mechanics. His taking seriously the reality of individual photons as discrete packets of energy in the photoelectric effect was groundbreaking in 1905, and the blackbody spectrum named after Planck could also be as easily called the Einstein spectrum, since Planck viewed it as an interpolation and a 'fudge' while once photons are accepted as real thanks to Einstein, the Planck spectrum for blackbody radiation can be derived.
Einstein made other major contributions to quantum theory, such as the A and B coefficients of spontaneous and induced atomic transitions, and his understanding that wavelike properties of matter essential to quantum mechanics would lead to Bose-Einstein Condensation, now beautifully confirmed in cold atom experiments.

So Einstein's relation to quantum theory was complex, and he thought more deeply about the issues than he is given credit for. How could he be completely 'against' a theory he had helped develop from its beginnings? What he was against is the Copenhagen interpretation of quantum measurements that Niels Bohr propounded. And here actually I think Einstein has by far the better argument. Anyone interested in this subject should read the correspondence between Bohr and Einstein, in which Bohr makes quasi-metaphysical arguments about duality, and Einstein sticks to a very rational train of thought.

Nor does Bell's theorems and modern experiments disprove Einstein in favor of Bohr. They show only that a very simplistic set of local hidden variable theories are inconsistent with quantum mechanics and experiment. But Einstein would by no means have to have been committed to such theories. Quantum correlations or 'entanglement' are quite real, and Einstein would have had to come to terms with that. No doubt he would have, concluding that the Lord, never malicious, is quite a bit more subtle than even Albert thought.

"How could he be completely 'against' a theory he had helped develop from its beginnings?"

A simple answer: because his cute little baby had developed into what he perceived to be an ugly irresponsible teenager.

"Nor does Bell's theorems and modern experiments disprove Einstein in favor of Bohr."

This is plain wrong. Einstein's realism is experimentally falsified. Don't let Bohr's vagueness stand in the way of the clarity achieved on this issue. I suggest you have a look at Mermin's thoughts on this subject.

Yes but that is not what Emil said. Emil said "Quantum correlations or 'entanglement' are quite real, and Einstein would have had to come to terms with that."

Einstein was reluctant to abandon traditional physical properties. QM appeared to be handing out spooky action at a distance (on those traditional variables) not to mention "this damned quantum jumping" (of those traditional variables). Einstein and Schrodinger didn't quite make the connection - that phi is the state of the whole system, observer and environment included - and considered that the theory was flawed. As nobody else made the connection either, it seems hardly fair to single them out as getting it wrong. But others then decided that rather than fix the theory (or, as it turns out, learn how to apply it wholeheartedy) they would attack the very idea of reality instead.

Schrodinger called the Copenhagen (non) interpretation "a philosophical extravaganza born of despair in the face of a grave crisis". It still is, and, on a personal note, this is the main reason I parted company with physics after graduating in what seemed to be little more than solipsism wrapped up in mathematics.

"Emil said "Quantum correlations or 'entanglement' are quite real, and Einstein would have had to come to terms with that."

Yes, and that is what I said in the above blog post. I postulated Einstein would have made the turn as soon as the Bell theorem got published, Emil might be more optimistic on the time Einstein would have needed - we will never know and according to modern insights, this non-fact is not even an element of reality :-)

I agree that Bohr was more often than not vague and cryptic. (In fact he was so difficult to follow that the Bohr-Einstein debate required an interpreter in the form of Ehrenfest.) Where we seem to disagree is in whether he was right.

Pity you left physics for the reason you mention. I think you must have been in the wrong physics class!

according to modern insights, this non-fact is not even an element of reality :-)That is why plain English has the word "would": to indicate things that "would" have happened "if" something else had been different. It's the falsehood of the condition (if Einstein had heard of the Bell equality) that disbars its consequences from being real. This doesn't seem to cause a problem for most people. Not sure why physicists have to be awkward about it :)

" 'Nor does Bell's theorems and modern experiments disprove Einstein in favor of Bohr.'
This is plain wrong. Einstein's realism is experimentally falsified. "

Sorry Johannes, but I think you are just helping to make my point.
A broad (mis)interpretation of 'Einstein's realism' was ascribed to him
by the Bohr school, a crude version of which you basically also
ascribe to Einstein in your article.

Bell's theorems and the experiments confirming QM show only that
a particular form of local hidden variable theories is inconsistent with QM.
To my knowledge Einstein never explicitly advocated those theories, which were
developed instead by people like David Bohm, perhaps inspired by Einstein's
thinking, but it is not correct to ascribe those theories to Einstein himself.
So their disproof does not disprove Einstein's view that there must be something
behind the successful recipes of QM, a deeper reason why they work.

I would be interested in any reference where Einstein says clearly in print that
any prediction of QM will be in disagreement with any experiment. If not, then
to the extent modern experiments are consistent with the predictions of QM they are also
consistent with Einstein's view that QM would give correct answers to limited questions
of a statistical variety, but was still not perhaps the whole story.

Finally arguing about what Einstein himself believed or did not believe is of historical
or biographical interest only. The physics question is whether the Copenhagen interpretation
of measurement and 'collapse of the wave function' is actually 'correct.' I would say that
the modern experiments show that it is not, since no energy or information is actually
transferred from one half of an EPR measurement of photons to the other half in real time.
The Bohr interpretation has burdened us with a great deal of words which obscure rather
than clarify the situation, which is one reason this topic is still debated on blogs like this.

Quantum correlations are subtle (as Einstein would have readily appreciated), and the
final word has yet to spoken on this topic. In the meantime his criticism of Bohr's
near mystical interpretation of the non-realism of QM remains penetrating
and cogent today.

Bell's theorems and the experiments confirming QM show only that a particular form of local hidden variable theories is inconsistent with QM.

-- 'Only' the sensible (local / non-contextual) hidden variable theories can be excluded as being inconsistent with reality. These were the theories advocated by Einstein. Einstein was wrong, but the mistake he made is still instructive. Of course you can create a hidden variable theory that describes Einsteins chest with socks. (Give it a try, it's an instructive exercise!) However, it will be a hidden variable theory at least as crazy as quantum mechanics.

Restricting ourselves to the historical question of what Einstein advocated, the most definitive statement
of his views that I am aware of is the famous EPR paper with Podolsky and Rosen. This paper does not
advocate any particular hidden variable alternative to QM, much less does it argue that QM predictions
for any experiment will be wrong. That's why Pauli and others had the view that Einstein's objections
were purely philosophical, with no predictive content different from QM itself. Bohm and later Bell managed to construct restrictive local hidden variable theories that would differ from QM, and these were falsified by experiments, but attributing these to Einstein after his death is questionable at best.

Instead the EPR paper essentially argues that even a theory that makes all statistical predictions correctly
like QM is nevertheless 'incomplete,' because it only predicts statistical averages, and cannot predict the result of any particular experiment. This is also why I say that Bell's theorem and modern experiments consistent with QM do not prove Einstein wrong.

Incidentally, I also heard a talk by a historian of science at a recent APS meeting in which he recounted
the details of how the famous EPR paper was written. It seems that Rosen (I think) talked with Einstein about it, then went off and wrote the paper with Podolsky, sending the final version to Einstein who had
mostly lost interest by then. Years after it was published Einstein apparently criticized himself, commenting that he should have read it more carefully. So even this definitive statement of Einstein's views are not necessarily what he believed.

Also as an aside, Einstein was wrong about gravitational waves which he did not understand at first
were a clear consequence of his theory, and was later persuaded they were.

Hi,
Imagine a 2D plain world in which particles are little 3D spheres moving always within the plane, intersecting it at their halves, and moving all around. Then imagine also that these spheres are painted red (one half) and green (the other half). Finally, let's assume the spheres are always in rapid rotation, unless they are observed (= interacting with a detector of this 2D world), during this process rotation temporary stops. Wouldn't the 2D inhabitants of this world detect these particles as "having two states (green/red) at the same time while not observed, and only green or red when observed, with a probability of 50%"? So they would build up their "quantum mechanics" theory, ignoring there is another level of reality behind, where the particles are actually higher-dimensional objects containing all possible states.

I'm pretty sure the whole revelation about Bell's Theorem and its extension by Kochen-Specker is that "No, you can't build such a formalism because it doesn't exist!"

After reading this article I've been playing around with the simple explanation for KS theorem as seen on wikipedia (the "diagram with the colored boxes"):http://en.wikipedia.org/wiki/Kochen%E2%80%93Specker_theorem
If I understand correctly, this means that for a non-zero vector in a vector field (as constructed in the article) , there can be chosen a different base so that the same vector when represented by the new base is zero?
... So whether or not a phenomenon was measured or not is dependent on the choice of coordinates?

Also, another question - I'm still not at a high enough level of math to understand everything in the wikipedia article, but if I understand correctly the assumption is made that any kind of space in which quantum phenomena occur has to preserve certain linear properties such as superposition and scalability. This assumption then leads to the contradiction I already stated.

Is it then at all possible to imagine such a 'non-linear' space with a different set of rules that behaves close to linearly when viewed at a far enough scale? Or am I just making the same mistake all over again in assuming such a space would give us any new information?

Here's the no answer. Your idea just creates randomness, it does not account for correlations between observations. For that you'd need a big machine on top, forcing different spheres to rotate at the last moment so that the observations are no longer due to actual rotation of the individual spheres but are, basically, faked. This makes the idea of individual rotations superfluous as the machine does the job instead.

The yes answer is that this machine is the wave function! Not directly observable but inferred from what it does.

Bear in mind I'm using your picture as a metaphor, don't expect real QM to be much like rotating spheres! Therer are lots of rotations of course but in a different sense entirely!

Einstein realised that there was a problem with quantum theory - nonlocality - by 1909 (see "Quantum Theory at the Crossroads" by Bacciagaluppi & Valentini - online at http://arxiv.org/abs/quant-ph/0609184) . After that it was a question of waiting for everyone else to catch up. While Bohr was alive they never did: his quasi-mystical pronouncements were all that were allowed in the way of thinking about how quantum theory actually worked. It was known that philosophising about quantum theory was a bad move in an academic career, presumably because those with that level of expertise were supposed to be making bombs rather than indulging in philosophy.

Bohm, who didn't have the security clearance to make bombs, devised an interpretation of quantum theory which had supposedly been proved to be impossible. Everett left academia after his many-worlds ideas were rejected by Bohr. One can hardly blame Einstein for thinking that, if people thought seriously about the problem of nonlocality, then they should be able to come up with a local, hidden variables model.

After Bohr's death, Bell started thinking seriously about nonlocality in quantum theory (still probably not a good career move). I think Einstein's response, had he been alive, would have been 'At last!'. Bell started off as a supporter of Einstein's view, seeing that the EPR argument meant that Locality=>Determinism (see J.S Bell in "Sixty-two years of uncertainty"). However, as we all know, his final result was in fact showed that Einstein's hope was not to be.

Nice article.
In the end, hoping that I am not going to be scolded by Sasha here, I would really like to understand what are the options still logically on the table with regard to QM "interpretation".
1) Local realism is ruled out. Ok.
2) Locality itself is, in my understanding, ruled out as well, since non-local effects in Bell's "theorem" are ultimately applicable to observations. Right?

Now, what I am not aware has been ruled out in full, as apparently assumed in this post, is realism itself. Of course realism "Einstein-style" (without "spooky action at a distance") is indefensible, granted. But I tend to believe that in any case spacetime and mechanical observables like speed, position, etc. are not fundamental altogether, and that they may be emerging from, say, a lower level "reality" like, say, spin networks. If this were the case, it is not apparent to me that the ultimate theory may not be "realistic", even if not local and certainly not classical.

On the other hand, I have a lot of trouble in figuring out a non-realist model of physical knowledge. If there is nothing "out there" to make our observations consistent and "continuous", if it is forbidden to speak of an "electron" while I am not measuring its properties, etc., how will I connect my observations, coming from different experimental setups, into a coherent knowledge? If I am using electron charge to measure its mass, I am implicitly correlating results from two experiments with incompatible (= with non-commutative autostates) setups, and if I do not accept the idea that the electron "has a property" like its electrical charge unless I am measuring it, I do not think I personally am able to even think of the physical world, let alone extract theoretical implications from the outcomes of an observation. But this could be my own epistemic limitation.

These (and other questions in this thread) are good questions. I don't have time to answer all of them, yet the comments to this article help me better understand the issues in conveying the 'quantum entanglement message' to a wider public. I think I am getting enough material for a follow-up blog post. Hopefully that will answer many of the remaining questons (and, no doubt, trigger new questions.)

In the meantime, let me try to eliminate a misunderstanding:

"Locality itself is, in my understanding, ruled out as well, since non-local effects in Bell's "theorem" are ultimately applicable to observations. Right?"

No. Locality is not ruled out by Bell and Kochen-Specker violations. Realism itself is ruled out. In Albert's chest terms: there is no non-locality (such as socks jumping instantaneously from one drawer into the other at the very moment you open a drawer), rather there is non-existence of displayed content of drawers not opened. You simply can't argue "If I would have opened three horizontal rows, I would have observed an even number of socks in total." There is no element of reality associated with 'measuring the content of all nine drawers'.

I look forward to your next post. Still, my understanding of Bell's "theorem", as published in "Speakable and Unspeakable in Quantum Mechanics", is that it tackles realism and locality, not realism per se.
Therefore, it should be possible to defend some form of non-local realism, just the way Bell himself did.

KS theorem does hit realism in itself, however, and I understand it poses very strong constraints on any tentative realistic theory. This, and the "emerging spacetime" hypotesis that I was mentioning before, could yield something "at least as weird as QM", to use your own words. Should you find the time to elaborate on this, I would be very interested.

I am a little puzzled about the complaints here of Bohr's mysticism. I thought he was completely pragmamatic.

If a click in a detector is dependendent on the entire experimental setup including the last second positions of mirrors and other objects in the aparatus, then the click is an expression of the total (global) distribution of all of the parts involved. If the insertion of one device precludes at a pragmatic level the insertion of a different device, then it only make sense that data from one setup is going to be different from what one could have instead collected from a mutually exclusive setup. I fail to see any hint of mysticism in this. Would not hidden variables or extra dimensions or a many worlds version be far more "mystical"?

Another way of viewing this is to contemplate an inverse mathematical map of the data. The more familiar direct-map is to start with a well-prepared setup and from that derive the experimental results. The inverse map starts from the experimental results, and asks what range of possible preparations could have produced the data. The beauty of quantum mechanics is that it never infers more than what it must, so from a particular set of data, entire equivalence classes of possibilites are left open as potentially being able to produce it. This incompleteness in being able to retro-dict need not be viewed as a defect or shortcoming in the theory as if there needs to be something beyond it.

As was noted by Richard Sheridan, in a different context and in a toast in 1802, “The glorious uncertainty of these laws was a thing well known and complained of, by all ignorant people, but all learned gentlemen considered it as its greatest excellency.”

For Bohr himself, his pragmatism was/is a logical extension of Einstein's vision:

“a comparison of purely logical aspects of relativistic and complementary argumentation reveals striking similarities as regards the renunciation of the absolute significance of conventional physical attributes of objects.” “... complementarity may be regarded as a rational generalisation of the very ideal of causality.” “In relativity theory, the emphasis on the dependence of all phenomena on the reference frame opened quite new ways of tracing general physical laws of unparalleled scope. In quantum theory, it was argued, the logical comprehension of hitherto unsuspected fundamental regularities governing atomic phenomena has demanded the recognition that no sharp separation can be made between an independent behavior of the objects and their interaction with the measuring instruments which define the reference frame.”*

*"Discussions with Einstein on Epistemological Problems in Atomic Physics" in Albert Einstein: Philosopher-Scientist, Cambridge University Press, 1949. This would have been at the time of Einstein's 70th birthday. As Bohr indicates, “The epistemological problems touched upon here were more explicitly dealt with in my contribution to the issue of Naturunssenschaften in celebration of Planck's 70th birthday in 1929.”

it only make sense that data from one setup is going to be different from what one could have instead collected from a mutually exclusive setup. I fail to see any hint of mysticism in this.

Agreed. I personally have no quarrel with Bohr, not having studied his writings myself. I have a quarrel with those who extrapolate the idea of mutually exclusive data sets to the non-existence of reality between measurements. It's not really about what the man wrote but what gets taught in his name. In particular the sudden collapse of the wavefunction is now dead in the water. The Copenhagen-ites were so adamant that it was not a determistic evolution of the wavefunction and now we know it is, it just happens very fast for precisely the reason you say. And the randomness comes from the fact we don't actually have any control over the phase of the macroscopic bits that do the measuring!

Would not hidden variables or extra dimensions or a many worlds version be far more "mystical"?

Well, that's the point. Having established limits on what we can measure, is it reasonable to assume there's a reality "out there" even when we are not observing it? The big unobservable wavefunction of the universe idea seems to make all the other interpretations excessively metaphysical. Does the fact I can only observe a shadow mean I must not speak of the thing itself? I would have thought that every time QM is vindicated and some fancy inequality is confirmed :) it just means that yet again, ouir reality is just the wavefunction observed by... wait for it... us, who are part of it. 'Scuse the sloppy language - YKWIM.Correction! I said " the randomness comes from the fact we don't actually have any control over the phase of the macroscopic bits that do the measuring! ". This is wrong. You can't get randomness out of the system even with environmental interaction. All you can do is include the observer in the system and then you get observer-specific outcomes. So it's Many Worlds all the way down. Sometimes life sucks.

In my opinion, all the problem about effect of observer on existence of observables are enlarged by the fact we put in the role of the observer a man, and not just a physical object with particular charateristics.
For nature there is no difference from the interference caused by a man or a photon or something else, in the sense that every interaction is ruled by the law of physics, without no use of terms about "knowledge", "memory", "intuition" or else.
If we start to think about measurement operation in that way, all the disturbing effect of quantum mechanics vanishes. All we get is two systems interacting (observer and observables), and just the rules about getting measurement results.
If we continue to think state vectors are real objects we cannot go out the dualism realism-quantum world. State vectors don't exist, all we get real are square modules, that is probability. The way state vectors moves and interact is all abstraction. That occurs even in classical mechanics. Any of you have ever seen a force vector? Only because the classical vectors are easier to connect to reality, we think that are more real than quantum state vectors. But both of them are abstract objects. So, in the abstract model (well working I think), of quantum mechanics, information can be put out of physical system only through measurement, and by disturbing the system in a non reversible way.
If we move from realism to information theory, we have no problems in understanding quantum mechanics.
In the abstract model the way to justify the loss of information during the measurement, to understand the differences by classical and quantum objects, and collapse of state vectors is using the concept of "decoherence".
Surely I've been too rough in my comment, but the space here is so limited to treat a so delicate argument.

Last time I looked, an abstraction was something like a number. The concept of "five socks" is made of real "socks" and the abstract number "five". It is exactly the same "five" as the "five" I get when I count the digits on one hand. It's been removed - *abstracted* - from any real-world context.

There is no way you can say the wave function is an abstraction. It plays the same role in physics as a force, a handy mental prop. To assert that it is real is not pathological, it is merely a metaphysical belief that something which explains everything must either be real or be a map to something else that is real.

Anyway, I'm trying to be constructive here, and not succeeding very well...

Sorry to be pedantic, but doesn't it just come down to semantics in the end, that is to say in particular on the meaning attributed to "observed"

Can an event be observed, if it has been detected by an instrument (which surely is the sense of the word in quantum physics) but not yet recorded by a human agent?

If one were to put CCTV cameras in the drawers......

As to my take on what exists or doesn't, I would say that no coherent thery exists, and arguing by analogy all the time is not getting anywhere because just how like are socks to photons?

The nature of things that don't exist can easily be described as a metarepresentation, Unicorns being the usual example. Do Unicorns have an impact on the world? Perhaps the idea of a Unicorn does even if the thing itself does not actually exist to confirm the impact of that idea?

If there exists neither a language nor a mathematics to describe the nature of the universe as it exists without these things, then what does/has/will exist other than some wierd compulsion on my part to post this?

Well, the double slit experiment tells right this. If you turn on a light source near one of the two slits, the interference goes away (experimentally proved).
If you want, you can say that all can interact, exists.
About unicorns one can use the Occam's razor to state that unicorns, without observation (direct or not) are not necessary to explain the world so we think that they (at the moment) doesn't exists.

Ah, just when I had made my peace with Copenhagen, you come along and stir up the old Einsteinian passion for deterministic locality. So I'm back to banging my head again, this time against that crazy sock chest.

OK, so under my crude understanding (or crude mis-understanding, or superposition of both), the individual drawer in the chest is the most basic thing there is. You can only measure any particular drawer though an interaction with another individual drawer. So, Einstein is blindly throwing drawers at the structure, but some of them are bouncing back at him and they tell him things about the drawer that he just hit. They can tell him how many socks are now in the drawer that was hit; or they can tell him something about the sock dynamics, its 'sock momentum'. But, they cannot tell him both things at once. He can never know simultaneously what the current sock state and sock momentum of the sock drawer is at any particular point in time by bouncing drawers off that drawer. I.e., if I'm not totally misunderstanding the whole Heisenberg thing (which is very possible).

So, under my Bohmian, wanna-hold-on-to-realism approach, Einstein, the blind man with nothing but a pile of drawers to throw at the drawer chest, cannot really examine any particular drawer and rule out some sort of sub-quantum (err, sub-drawer) mechanism that responds to his choice of row-versus-column sequencing. And just what the heck is it in the chest that locks up certain drawers after the first one has been pulled, anyway? The realist in me sez, hey, SOMETHING is in there doing this. And if only we had tiny little bundles of energy with which to shoot through this chest and get some images of its inner workings (like x-ray photons), we might be able to observe and understand it.

But we don't. Get used to it, all we have are these clunky drawers to throw at the individual drawers, and they can only tell us one thing about the drawer in the chest, depending on how we throw it. We can't accumulate enough controlled information to understand just how this chest-drawer system always gives us evens one way and odds the other.

Sigh. What a world -- even if as realistic / deterministic / localist as I describe above; it could be even worse! So, get out the philosophy tomes and let's bang our heads some more on the unanswerable questions, e.g. if a tree falls in the forest and no one comes to cut it into sock drawers . . . We live in a world that is an information prison, walled in on one side by light speed, and on the other by quantum size and indeterminacy. And let's not even get into emergent semi-deterministic chaos in large-scale, complex non-linear recursive systems!!

But one other possibility . . . what's so bad about the notion of time being more than 1D? So what if the future somehow entangles with the present (and the past, while we're at it), so long as information is not conveyed? Ay, but there's the rub, isn't it. It's just a thought, one that cannot ever be empirically verified. It's that hypothetical tree in that hypothetical forest that no one can ever visit . . . the tree that got knocked down by the pact of unicorns. Oh well . . .

And yet, four walls do not a metaphysical prison make. Especially if you are a quantum wave-particle, and there's always a chance that you will "decohere" on the outside!!

Just some half-baked, half-ass, half-understood thoughts, feel free to correct!

I shouldn't get too frustrated, Jim. You may feel walled in on one side by light speed, and on the other by quantum size and indeterminacy, but the reality is we are all computer simulations - as Hilary Putnam has amply demonstrated.

I'm really sorry for the presence of so much terms to speak about physics (theoretical in particular), but it's not my fault if we need ALL of them.
When you make the example of five you are jumpin out a formal passage of crucial importance. The number five doesn't exist as something real without a function that connects it to reality.
You can instead define it as an abstract object as the number theory does but we don't care about that in this discussion.
So you make the theoretical association between a characteristic of a real object (numerability) and the number five, then you compare two sets of objects and see that the parameter number is the same.
It's the same thing you do with vectors, even state vectors. A force vector doesn't exist as a real object if you don't define a relationship between it (module, direction and verse) and parameters (observables) of a physical system.
The same with the state vector, you have a only function that connects it to reality (observables) and that's "the square module is related to probability". You cannot tell nothing more. So all the others parameters are abstract and live in abstract world, until you arrive at square module and talk about reality.
Do you think that Schroedinger equation is something real? Absolutely not. It's a rule of the game of abstraction we have found that is well related to observables values. If it was real, how to explain the progress of theories? Reality can be only one, isn't it? So theory is good only if we make the right choices of function that relates math to reality and only if we define the domain where it works.
If we start to think that math tools are real, well every contradiction is welcomed. Wave function doesn't exist because there's no evidence of them. If you think they are real, you are making the same error of Einstein with the theory of "ghost waves". If they were real, how do you explain the wave-particle dualism? Are electrons moving on this kind of waves? Are they spread on them? How can electrons in wave form interfere with themselves?
Repeat, I'm sorry for being rough in my explanation but it's a really delicate argument to be treated fully here.

They are all weasel words. Some have precise definitions that make them useable, but if you take, for example, "World" out of its context of a superposition, you are left, at best, with science fiction and, at worst, gibberish.

Do you think that Schroedinger equation is something real? Absolutely not. It's a rule of the game of abstraction we have found that is well related to observables values.

What I said was:

To assert that it is real is not pathological, it is merely a metaphysical belief that something which explains everything must either be real or be a map to something else that is real.

A map to something else. Not necessarily a physical entity but a true statement about the universe. That is exactly what you have just said about finding a mathematical form that matches what nature tells us. What made you think I needed it spelled out when I'd just said it myself?

Finally, please do not say I am making the same mistake as Einstein! I would be delighted if I could be meaningfully compared to Einstein even in a mistake, but I fear that mentioning him and me in the same sentence is more of an insult to Einstein than a compliment to me.

Einstein was not mistaken to try to find where Schroedinger's equation relates to reality. He fully understood all the alternative theories and opined that there was something incomplete about QM. Einstein did not modify QM even when he, Podolsky and Rosen discovered entanglement lurking in the maths. Bohr and Heisenberg, however, did modify the theory, right from the beginning, the moment non-commutivity was found - also lurking in the maths. They insisted that a system takes a break from obeying Schrodinger's equation whenever someone looks at it; it then selects one of the eigenstates and rearranges itself to comply - in total disregard for every conservation law known to physics. This theory was disposed of once environmental decoherence was understood and confirmed by experiment. Bohr and Heisenberg were totally wrong. They had mistaken the "isolated system" approximation for a statement about the "total system" and modified the theory to make the approximation apply even where the system is described exactly.

Still, at least they were trying to take the Schrodinger equation seriously, if only by radically contradicting it. Schrodinger, of course, hated the thing, being smart enough to realise that if the maths takes a wavefunction collapse, then something in the real world must do too. The interference implied by QM, both on-axis (observable) and off-axis (entanglements) is experimentally confirmed. Therefore it refers to something "out there". It really is quite irrelevant whether you call the pencil and paper version an abstraction or not - your original claim was this:

If we start to think about measurement operation in that way, all the disturbing effect of quantum mechanics vanishes. All we get is two systems interacting (observer and observables), and just the rules about getting measurement results.

In other words "shut up and calculate"! Not good enough. Any solipsist can ignore the real world around them until you stick a pin in him. This is a descent into applied technology - useful if you just want to make a better iPad but rubbish if you are interested in what the world is really like. Perhaps I blinked, but that was what science used to be about, I don't remember anyone announcing that its objectives had changed.

I'm sorry you take on personal the present discussion.
What I'd like to assert is that every thing in theory is abstraction and has 'some' connection with reality (what reality means for physics is everything on which one can do experiments and measures).
In classical vectors we put this relationship for example between module of force vector with the quantity called intensity of a force, so whatever operation you do on vectors has a related experiment on reality.
But adding vectors is not a 'description' of how classical forces works, we can say that only because experiments confirm that values, but it's all abstraction.
So we have state vectors, they have some relationship with reality, but entanglement doesn't have a direct connection with experiments, you never see an entangled state. This is because they are in part abstraction. It is the way how information interacts and propagate through the mathematical system, but when you try to understand if it's right or not you have to see experiment says and there only square modules talk.
What realist want to do is not to relate theory to experiments, because this is already done in qm, but try to give a reality to state vectors, or wave functions, as if we can measure directly them.
No, state vectors, as force vectors, are tools, are the way we can describe (get numbers related with experiments) how nature works. You cannot be satisfied by this description, if you will be able to give a better description I'll be the first to congratulate with you. But remember that the description has to describe the double slit experiment and photoelectric effect together. Sincerally yours, Paolo.

You cannot be satisfied by this description, if you will be able to give a better description I'll be the first to congratulate with you. But remember that the description has to describe the double slit experiment and photoelectric effect together.

Why on earth should not being satisfied with one description mean I have an alternative to offer?

Mind you, you example is a bad one. The wave/particle duality can be be explained rather easily if the interaction is non-local - in particular a particle can go through a single slit and remain a particle - as long as momentum is exchanged according to the spatial components of the diffractor. Which is exactly what it should do given the conjugate realtionship of space (position) and momentum. The maths comes out the same and with the same degree on non-locality, the picture, however, stays with particles, no waves and especially, no wavicles. Other quantum phenomena are not so simple of course.

Hello Johannes,
I am a humble Dutch crackpot.
May I suggest an extended Stern Gerlach experiment with entangled vaporized silver atoms, which could perhaps give a sort of an answer on the a problem that John Bell with the S-G experiment had?

New Stern Gerlach experiment, based on the hypothesis, that the so called "non-smearing paradox" described by John s. Bell is originated by a new form of Entanglement.
See J.S.Bell's book: "Speakable and Unspeakable in Quantum Mechanics"(Ch16,p.140-141, first edition)
The new entanglement seems to be present between the first silver atom entering the S-G magnet and all the other vaporized silver atoms inside the oven or under way into the S-G Magnet.
If the first atom is polarized by the magnet, the rest immediate seem to rotate in up or down position, even before entering the S-G magnet.
As a result only up and down positions on the screen are observed.
Only the first measured atom is supposed to have an at-random polarization and the opportunity to become an intermediate or "smeared" position on the screen.
This would be the logic explanation, to solve John Bell's non smearing paradox.
Thus it is assumed that all the vaporized silver atoms are in an active entangled state during measurement of at least one of them.
So if we could do the experiment with TWO opposing S-G magnets with 90 degree mutual rotation, a BAR should be present inside the screen picture, caused by the perpendicular spin state of the alternating first atom that enter one of the magnet
See perhaps:http://bigbang-entanglement.blogspot.com/2011/06/new-kind-of-entanglemen...

There's a good Wikipedia article on successive SG magnets. The point is very clear, the beam separates without smearing.

However, remember that the SG experiment was performed in 1922 and the physical theory was dodgy. Photons are easier to deal with. What's more they always align themselves with the axis system of the polarizer even if it hasn't been set yet. In fact the polarizer needn't exist. Just thinking the word "vertical" makes every photon in the universe instantly enter a superposition of states ready to interact with a polarizer...

In other words it's a mathematical trick not requiring any more physical explanation that the fact you can say "10 = 5 + 5" or "10 = 3 + 7". A device like a magnet or a polarizer is only required if you want to make a measurement and then the dynamics are fully explained by decoherence.

Apparently not. I don't have his book handy and you haven't explained the point at all clearly so it's more than possible I have guessed wrongly. If it's just about confusion in 1922 about the spin states on a silver atom, then forget it! Perhaps you would like to put it in your own words?

I started with Lawlers dual particle concept which immediately questions Maxwell. The consequences have been interesting. Fields do not exist at all. Everything, including gravity, is a consequence of particles. While it explains more, it raises other questions, particluarly at the atomic level.

Thanks Derek,
I am sorry, I am away from home and John Bell's book for 2 weeks,
Perhaps somebody else is able to copy the text,
"Speakable and Unspeakable in Quantum Mechanics"(Ch16,p.140-141, first edition)
John Bell called the smearing phenomenon: the "absence of intermediate deflection" of the silver atoms on the screen..

I happen to have the first edition of Bell's book. Chapter 14 is entitled "Bertelmann's socks and the nature of reality". (What is it that causes theoretical physicists' fetish for socks?) Reading the section you refer to, I get the feeling that Bertelmann's socks don't add to the point made with Albert's socks. Here is the text (I think) you are referring to:

A particle of given species is supposed to have a given magnetization. But because of the variable angle between particle axis and field there would still be a range of deflections possible in a given Stern-Gerlach magnet. It could be expected then that a succession of particles would make a pattern something like Fig. 4 [linear smear] on a detecting screen. But what is observed in the simplest case is more like Fg. 5, with two distinct groups of deflections (i.e., up or down) rather than a more or less continuous band. (This simplest case, with just two groups of deflections, is that of so-called 'spin-1/2' particles; for 'spin-j' particles there are 2j+1 groups).
The pattern of Fig. 5 is very hard to understand in naive classcal terms. It might be supposed for example that the magnetic field first pulls the little magnets into alignment with itself, like compass needles. But even if this were dynamically sound it would account for only one group of deflections. To account for the second group wuld require 'compass-needles' pointing in the wrong direction. And anyway it is not dynamically sound. Thie internal angular momnentum, by gyroscopic action, should stabilize the angle between particle axis and magnetic field. Well then, could it not be that the source for some reason delivers partcles with axes pointing just one way or the other and not in between? But this is easily tested by turning the Stern-Gerlach magnet. What we get (Fig. 6) is just the same split as before, but turned around with the Stern-Gerlach magnet. To blame the absence of intermediate deflections on the source we would have to imagine that it anticipated somehow the orientation of the Stern-Gerlach magnet.

My suggestion is, that the first entering "calssical"silver atom is turned around by the SG magnet more or less into a magnetic alignement with tje SG magnet and because is has what I call "source entanglement" with the silver oven atoms, it will polarize these atoms before they enter the SG magnet.
Thus the first "more or less aligned silver atom has the possibility to be smeared. but the second and the third , are supposed to to be pre-aligned already by this source entanglement, before entering the SG magnet.
So , this is a semi classical explanation including a new kind of entanglement..
Therefore I suggested to do a test with a second SG magnet at the opposite side of the first magent.

Yes, I can do better than Einstein. I guess people already don't like my answer. I derive the whole of physics from a simple postulate " Reality is a Mathematical structure". It is the only dynamic structure possible using fundamental entity which is a random line.

Not only entanglement is explained. It is the source of everything including the mass of the particles. Spin directly decides the mass of the electron.

Particles are modelled as end of lines, one end is confined to a small region and the other to other particles in the universe. Let do that in 2D. I simulate two particles interacting by counting the random lines that cross each other from the two particles. in the X-axis I get the 1/r law for coulomb potential.

NOW for the big surprise. In Y-axis the particles can only interact on a region set only by their width and it is NOT dependant on the distance between them. so each on can be at the other side of the universe. and the expectation value are always the opposite of each other, one is negative the other is positive. What more can you ask for.

I would say that I have a baseline understanding of this, but very limited. My question is if 'things' in the non-quantum world are made up of 'things' in the quantum world, why can't/don't non-quantum objects follow the same rules? If I am walking in a virgin forest and come across a tree that is laying on its side, should I assume that it did not fall but came into existence that way as soon as I observed it?

So we are being brow-beaten into thinking. I think it's a level of sophistry which replaces the old quantum mysticism of magical observers with a new one where there is a diffuse kind of non-reality about everything unless you actually observe it. And even then it's only real "to you". Which is about as close to postmodernism as physics can get :)

In MW, of course, the observation is not so important, the tree and you can always exist if your primitive little "realist" mind wants to believe it, but real (tm) physicists only know about what you can observe, and in Many Worlds there are many observers observing many things: different versions of you observing different versions of the tree, including the one in which, by pure quantum bad luck, it gets up and does a seductive dance in front of you.

It gets better. In MWI, the different worlds separate, for no obvious reason, and although only present at a level of say 1 googolplexianth in the state that you (or rather this version amongst countless other versions of you) entered the forest, the crazy world of the dancing tree suddenly becomes fully normalized so that in its own branch (unfortunate term) it is just as real as all the others. Lots of wierd worlds...

Yup, you can spin it however you like. Physics had a nice breakthrough when people actually started taking decoherence seriously but it seems to have wandered off into Lala land again. Einstein must be rolling in his grave.

My personal conviction is that the process of wave function collapse of two entangled particles into mirror symmetrical states, is the basic process for all created particles, then we should live inside one part of a mirror symmetrical entangled copy MULTIVERSE with one central origin.
We could give it the name: Raspberry Multiverse.
Even human choice making should be subjected to this mirror symmetrical process.
Benjamin Libet described a choice experiment, which we could use to measure how many copy universes there should be.
Then we could live inside a symmetrical 12 fold dodecahedron shaped bubble multiverse?

The Copenhagen interpretation says that we HUMANS are the unique observers of a unique universe.
However, if we live inside one of the two (or even more) mirror symmetrical entangled Copy
universes, then the problem is solved, because in that case there is no uncertainty any more
about the cat at the quantum level or our own choice making.
Then, at the quantum level the choice is already made about the collapse of the wavefunction between the mirror symmetric quanta by the entanglement relation between each set of quanta, even if these quanta are separated at huge distances.

If in our material universe the cat stays alive, he also will live in the mirror anti-material universe, caused by the mirror symmetry of the entanglement relation. In this way those mirror universes will always stay each others mirror copies even at the quantum level.

However, as a consequence, it is a shock to realise that I am not alone inside the Multiverse, there must be at least ONE entangled COPY of ME who also tries to make decisions and is influencing me.
Benjamin Libet’s experiment on human decision making could help us understand this phenomenon. Libet measured electronic brain activity differences between Readiness Potential I and II, which he did not understand.
Future Libet-experiments focused on the statistics of the differences between RPI and RPII will be indications of the number of entangled universes inside our Multiverse.(see fig.6,http://vixra.org/pdf/1103.0015v1.pdf
)

I think there is a way out of this MWI nonsense. Or at least have a less appalling one.

Observing is interacting. So instead of talking about systems that have or have not been observed we switch to talking about systems that have or have not interacted with the rest of the universe.

The question at hand is, what is the history of a system between two interactions. Say, we create two photons and then catch them some time later. In between, these photons do not have any interaction at all. They just fly.

When we catch these photons, we can try to deduce what they did between creation and catching. There are several options of how reality could handle this question.

The realist option would be that the photons had some specific history during creation and flight, as if they were constantly observed.

However, quantum theory and practice shows that when caught, the photons behave as if all possible histories have played out.

That comes out because you cannot ask all questions at the same time. Some questions preclude others (polarization, two-slit experiment). Whatever question you ask (interactions are happening) will result in a consistent history. But all possible histories are available depending on the question you ask. In the Drawer of Johannes, there is no way the distribution of socks over the drawers will influence anything in the universe until someone looks. Therefore, all distributions are possible and available and there will always be a consistent outcome.

So, instead of Multiple Universes, you only have Multiple Possible Histories of that single system while it does not interact.

The nice thing is, this also applies to Schroedingers cat paradox. There is a quantum system of decay, which is invisible to the cat until the decay actually happens. Then the cat knows (briefly). But as the system does not interact with the rest of the universe, all histories are still possible to us until the box is opened. So this system will behave to us as if the cat was both alive and dead during the whole experiment. But it will behave to the cat is if it is alive OR dead.

Actually, I think this is just as daft an explanation as the MWI. It does cut the liens between observing and reality. You can only know what you have observed, and there is no reason to assume your observations disclose all of reality. And I simply like my own explanation better.